Small-scale Sr and O isotope variations through the UG2 in the eastern Bushveld Complex: The role of crustal fluids
Small-scale Sr and O isotope variations through the UG2 in the eastern Bushveld Complex: The role of crustal fluids
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DOI：
10.1016/j.chemgeo.2018.03.040
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发表时间：
2018-05
期刊：
影响因子：
3.9
通讯作者：
Mathias Schannor;I. Veksler;L. Hecht;C. Harris;R. Romer;T. Manyeruke
中科院分区：
文献类型：
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作者：
Mathias Schannor;I. Veksler;L. Hecht;C. Harris;R. Romer;T. Manyeruke
对布什维尔德杂岩体东部第 2 族 (UG2) 单元矿物的 Sr 和 O 同位素组成的详细研究揭示了伟晶状下盘辉石岩和 UG2 铬铁矿层本身的显着变化。下盘岩石中间质斜长石的初始 87Sr/86Sr 比值 (Sri) 显着高于临界区邻近岩石。下盘辉石岩和铬铁矿中斜长石-辉石和斜长石-铬铁矿矿物对的 δ18O 值分别与结晶温度低至 700°C 和 800°C 时的氧同位素交换一致，而在 UG2 的其他部分，斜长石和辉石 δ18O 值与液相线结晶温度约为1150°C。我们认为，UG2 铬铁矿和下盘辉石岩中斜长石的初始 87Sr/86Sr 较高，反映了 Bushveld 杂岩下脱水围岩产生的流体的渗透。流体可能在铬铁矿结晶后期或过程中引入。铬铁矿层及其伟晶状下盘可能充当了截留晚期地壳流体的屏障。或者，流体与含铬铁矿的结晶糊层的反应将引起辉石和斜长石的水合熔化，向熔体提供Cr和Al，从而引起进一步的铬铁矿形成。这种溶解反应从辉石中释放出铬，与预先存在的浸染铬铁矿一起可以细化铬铁矿层。伟晶状的UG2下盘构成了这一过程的反应前线，在冷却时重结晶为粗粒岩石。这项研究强调了地壳流体和晚期和后岩浆过程在亚固相线温度下发生的层状侵入中的作用，从而同位素和化学成分可能发生显着变化。
A detailed study of the Sr- and O- isotopic composition of minerals in the Upper Group 2 (UG2) unit of the eastern Bushveld Complex revealed significant variations in the pegmatoidal footwall pyroxenite and the UG2 chromitite layer itself. Initial87Sr/86Sr ratios (Sri) of interstitial plagioclase in the footwall rocks are significantly higher than in adjacent rocks of the Critical Zone. The δ18O values of plagioclase-pyroxene and plagioclase-chromite mineral pairs in footwall pyroxenite and chromitite are consistent with oxygen isotope exchange from crystallization temperatures down to 700 °C and 800 °C, respectively, whereas in other parts of the UG2, plagioclase and pyroxene δ18O values are consistent with O isotope equilibration at liquidus crystallization temperatures of approximately 1150 °C.We propose that the higher initial87Sr/86Sr of plagioclase in the UG2 chromitite and the footwall pyroxenite reflects infiltration of fluid derived from dehydrating country rocks beneath the Bushveld Complex. Fluids could have been introduced either late or during chromitite crystallization. The chromitite layer and its pegmatoidal footwall could have acted as barriers to entrap late stage crustal fluids. Alternatively, reaction of fluids with the chromite-bearing crystallizing mush layer would induce hydration melting of pyroxene and plagioclase supplying Cr and Al to the melt, thus invoking further chromite formation. This dissolution-reaction releases Cr from pyroxene, which along with pre-existing disseminated chromite could refine the chromitite layers. The pegmatoidal UG2 footwall constitutes the reaction front-line of this process, which recrystallized to a coarse-grained rock upon cooling.This study emphasizes the role of crustal fluids and late- and post-magmatic processes in layered intrusions taking place at subsolidus temperatures, whereby isotopic and chemical compositions may be significantly changed.